The present invention relates to a thread cutting machine including a typical tapping machine, and more particularly, to a type capable of performing a thread cutting operation precisely even at high speeds.
In a prior technique, a thread cutting working in a thread cutting machine to which a numerical control (NC) apparatus is applied has been performed by feeding and rotating a thread cutting tool (or tapping tool) such as a screw tap in accordance with the pitch of the threads to be worked on the basis of instructions from the NC apparatus. A feed motor for feeding the thread cutting tool and a spindle motor for rotating the same have been controlled by servo systems independently of each other, that is, both motors have been controlled without mutual relationship. The discrepancy between the feed amount and the rotation amount of the tool which may be caused in a reverse rotation of the spindle motor is compensated for by the mechanical expansion and contraction of a tapper interposed between the tapping tool and the spindle. This controlling method includes such problem as that the machining speed of the thread cutting is limited by the performance of the tapper to be used and the thread accuracy of the thread cutting may be lowered by the expansion and contraction of the tapper.
In order to overcome these problems or drawbacks, in the prior art Japanese Patent Laid-Open (KOKAI) Publication No. 56-33249 the actual rotation amount of the spindle is detected and the feed motor is driven in accordance with the detected rotation amount. In Japanese Patent Laid-Open (KOKAI) Publication No. 60-155319, the actual feed amount (i.e. rate of advance) of the spindle is detected and the rotation motor is driven in accordance with the detected feed amount. These prior art publications provide controlling apparatus wherein the feed motor and the rotation motor are driven in a synchronous mode, and the controlling apparatus can attain accurate synchronism between the feed motor and the rotation motor so that the thread cutting operation can be usually achieved without using any tapper means.
Recently, however, it is strongly required to shorten the machining time, and in some case, to carry out the thread cutting operation at a high machining speed substantially equal to the maximum machining speed of the tapping machine. In such a case, in the conventional machine, of the type described above, for driving the rotation motor in accordance with the feed amount of the feed motor, a follow-up time lag is observed because the instruction to the rotation motor is issued after the actual displacement of the feed shaft has been detected, thus providing a limitation to the improvement of the thread working accuracy. Particularly, in the thread cutting operation with relatively shallow thread depth or stepping working of the tool with relatively small stepping width, the actual working is often carried out under the transient condition before the feeding and rotating speeds reach the constant values, thus providing a significant problem causing machining error due to the follow-up time lag.